In a contention based random access, an evolved Node B (eNB) does not allocate any dedicated resource for a User Equipment (UE), and a random access procedure is initiated randomly by the UE on its own initiative. The contention based random access is applicable to all of the five scenarios: for the scenarios of Radio Resource Control (RRC) connection establishment, RRC connection reestablishment and uplink data arriving, the random access is initiated by the UE on its own initiative, and the eNB does not have any priori information; and for the scenarios of a handover and downlink data arriving, the UE is indicated by the eNB to initiate the random access. Generally the eNB preferentially selects a non-contention based random access, and only if resources are insufficient to be allocated for the non-contention based random access, then the eNB may indicate the UE to initiate a contention based random access. The contention based random access is generally made in four steps as illustrated in FIG. 1, where each step is represented as a message, and these four steps are represented as Msg1 to Msg4 in the related standards.
(1) The Msg1 relates to a preamble sequence.
This message is an uplink message transmitted by the UE and received by the eNB, and it is transmitted by being carried in a Physical Random Access Channel (PRACH). For a contention based random access, the preamble sequence transmitted by the UE is one selected randomly in a specific set of preambles, and there are different identifies, i.e., preamble indexes, for different preamble sequences.
(2) The Msg2 relates to a Random Access Response (RAR).
This message is a downlink message transmitted by the eNB and received by the UE.
The Msg2 is a response of the eNB to the random access initiated by the UE upon reception of the Msg1, and shall be transmitted in a random access response window. The random access response window starts at the first available downlink sub-frame starting with the sub-frame n+3, where n is the number of the last sub-frame for transmitting a preamble, and the length of the random access response window ranges from 2 to 10 ms, and is particularly specified by the eNB in a system message.
The Msg2 is carried in a Downlink Shared Channel (DL-SCH) defined at the Media Access Control (MAC) layer, and corresponds to a Physical Downlink Shared Channel (PDSCH) at the physical layer. One Msg2 can respond to random access requests of a plurality of UEs, that is, it can carry RARs for the plurality of UEs, so there is no Hybrid Automatic Repeat Request (HARM) procedure for the Msg2, that is, there is no feedback and repetition transmission procedure for the Msg2.
The eNB schedules the Msg2 over a Physical Downlink Control Channel (PDCCH) scrambled using a Random Access Radio Network Temporary Identifier (RA-RNTI) and transmitted in a common search space, where the RA-RNTI is determined by the time-frequency resource position of a PRACH over which the Msg1 is transmitted. The UEs using the same PRACH time-frequency resource can get the same RA-RNTI from the calculation, and their RAR information is packaged into the same RAR message and transmitted, where the RAR message is carried in a downlink shared channel scheduled by the PDCCH scrambled using the RA-RNTI, and the downlink shared channel is also scrambled using the RA-RNTI.
The Msg2 includes a back-off parameter, the identification information of the preamble sequence corresponding to the Msg1, an uplink transmission Timing Advance (TA), an uplink (UL) grant (also referred to as an RAR grant at the physical layer) of the Msg3 and a Temporary Cell-Radio Network Temporary Identifier (TC-RNTI), where the back-off parameter indicates an average delay at which the UE initiates the next random access if the current random access fails.
The UE identifies the RAR message carrying the RAR of the UE (i.e., the Msg2 transmitted in the DL-SCH), using the RA-RNTI determined by the PRACH time-frequency resource over which the UE transmits the preamble sequence in the Msg1, and determines the RAR transmitted to the UE in the RAR message according to the identification information of the preamble sequence transmitted by the UE in the Msg1. If the UE does not receive the Msg2 correctly, then the UE may determine a delay for initiating the next random access according to the delay constraint in the back-off parameter, and further select a random access resource and initiate the next random access. After the largest number of random accesses is reached, the MAC layer of the UE reports a random access failure to the RRC layer to trigger a Radio Link Failure (RLF) procedure.
(3) The Msg3 relates to initial scheduling of uplink transmission.
This message is an uplink message transmitted by the UE and received by the eNB.
The UE performs uplink transmission over an uplink resource indicated by the UL grant (i.e., the RAR grant) included in the obtained RAR of the UE upon reception of the Msg2, that is, the UE transmits a Physical Uplink Shared Channel (PUSCH) according to scheduling information in the RAR grant of the UE, where the PUSCH corresponds to an Uplink Shared Channel (UL-SCH) at the MAC layer. HARQ mechanism is used for Msg3, and at least 56 bits can be transmitted over the uplink resource of Msg3.
Initial transmission of the Msg3 is the only uplink transmission scheduled dynamically at the MAC layer, and needs to be processed at the MAC layer, so there is an interval of at least 6 ms between the Msg3 and the Msg2. All of other repetition transmissions of the Msg3 are scheduled by a PDCCH, which is transmitted in a common search space and scrambled using the TC-RNTI.
(4) The Msg4 relates to contention resolution.
This message is a downlink message transmitted by the eNB and received by the UE.
The eNB and the UE complete the final contention resolution via the Msg4 (the contention arises from different UEs selecting the same preamble sequence and the same PRACH time-frequency resource to transmit the Msg1). The contents of the Msg4 correspond to the contents of the Msg3. The Msg4 is carried in a DL-SCH defined at the MAC layer, and corresponds to a PDSCH at the physical layer. One Msg4 can only respond to contention resolution messages of a set of contending UEs (actually only one of the UEs succeeds). A downlink control channel carrying scheduling information of the Msg4 is transmitted in a UE-specific search space and scrambled using the TC-RNTI or the C-RNTI, and which type of RNTI is used for scrambling is particularly dependent upon the trigger reason and scenario of the contention based access. The HARQ mechanism is also applicable to the Msg4, but only the UE decoding the Msg4 successfully and completing the contention resolution can feed back Acknowledgement (ACK) information; otherwise, no feedback is transmitted.
The UE starts a contention resolution timer (mac-ContentionResolutionTimer) after transmitting the Msg3, and restarts the contention resolution timer each time the Msg3 is retransmitted. If the contention resolution has not been completed after the contention resolution timer expires, then a failure of contention resolution may be judged. If there is a failure of contention resolution, then the UE may operate in a similar way to a failure of receiving the Msg2, where the UE determines a delay for initiating the next random access according to the delay constraint in the back-off parameter, and further selects a random access resource and initiates the next random access. After the largest number of random accesses is reached, the MAC layer of the UE reports a random access failure to the RRC layer to trigger an RLF procedure.
As the internet of things is emerging, a support of Machine Type Communication (MTC) in a Long Term Evolution (LTE) system has been increasingly recognized. An enhanced physical layer project for MTC has been set up in the 3rd Generation partnership Project (3GPP) Release 13. An MTC device (or an MTC terminal) may have a part of various Machine to Machine (M2M) communication characteristics, e.g., low mobility, a small amount of data to be transmitted, insensitivity to a communication delay, extremely low power consumption as required, and other characteristics, where in order to lower a cost of the MTC terminal, a type of terminal supporting only a 1.4 MHz radio frequency bandwidth in the uplink and the downlink will be newly defined.
In the existing networks, the operators have identified that for a terminal operating in some scenario, e.g., a terminal operating underground, in a shopping mall, or at a corner of a building, a radio signal is seriously shielded, and the signal is greatly attenuated, so the terminal cannot communicate with the network, but if a coverage area of the network is extended in such a scenario, then a cost of deploying the network will be greatly increased. Some test has showed that the existing coverage area needs to be enhanced to some extent. A feasible practice to enhance the coverage area is to apply repetition transmission or other similar technologies to the existing channels, and theoretically tens or hundreds of repetition transmission can be performed over the existing physical channels for some coverage gain.
Coverage enhancement levels of UEs need to be distinguished from each other in an MTC system by time and frequency resources and preamble sequences used for PRACH access. No repetition transmission is required for a normal coverage mode (the coverage enhancement level 0). There are different start instances of time of repetition transmission and different numbers of repetition transmissions for the PRACHs between the different coverage enhancement levels. There are also different start instances of time of repetition transmission and different numbers of repetition transmissions for the downlink control channels, scrambled using the RA-RNTIs, scheduling the random access responses in the Msg2 procedure, so the downlink control channels, scrambled using the RA-RNTIs, corresponding to the UEs with the different coverage enhancement levels may be partially overlapped on transmission times during the repetition transmission. If they are transmitted as in the prior art, then resource collision may occur, so that some UE has no resource to transmit its downlink control channel scrambled using the RA-RNTI, and thus cannot receive any random access response. The UE needs to obtain its scheduling signaling by detecting different downlink control channel candidates blindly, thus resulting in some power waste. At present, there has been absent a specific solution about how to avoid resource collision between the downlink control channels, scrambled using the RA-RNTIs, corresponding to the different coverage enhancement levels. Alike the downlink control channels, scrambled using the TC-RNTI, scheduling the Msg4 may also partially overlap in the Msg4 procedure, and if they are transmitted as in the prior art, then resource collision may occur. The downlink control channel can be transmitted in the UE-specific search space in the prior art, but configuration information of the specific search space is unavailable for an initial random access in MTC, so that some UE has no resource to transmit its downlink control channel scrambled using the TC-RNTI, and thus cannot receive any contention resolution message.